1. Field of the Invention
The invention relates to a method for selecting a transmission channel for transmission of messages from a mobile terminal to a base station, in which the terminal initially sends a send authorization request signal for a specific transmission channel to the base station and the base station then sends a response signal to the terminal, which contains a first decision value, with which the terminal is signaled whether or not it is authorized to send a message on the requested transmission channel. In addition the invention relates to a corresponding terminal and a base station which can be used to carry out such a method.
2. Description of the Related Art
In cellular mobile radio systems a communication connection between a mobile terminal, also referred to as a terminal, a mobile radio device or “User Equipment” (UE) below, and the mobile radio network is established via a base station which serves the mobile radio subscribers in a specific group—in what is known as a cell—via one or more radio channels. Such a base station—in the UMTS (UMTS=Universal Mobile Telecommunication System) also known as “Node B”—provides the actual radio interface between the mobile radio network and the mobile terminal. It takes over the handling of radio operation with the various mobile subscribers within its cell and monitors the physical radio connections. In addition it transfers network and status messages to the terminals.
Such a base station must feature at least one transceiver unit with suitable antenna device as well as a processor device, which control the various processes within the base station. Likewise the individual terminals must each feature corresponding transceiver units with suitable antenna devices and corresponding processor devices in each case which control the various processes in the relevant terminal.
In mobile radio a distinction is made between two connection directions. The forwards direction (Downlink, DL) describes the direction from the base station to the terminal, the backwards direction (Uplink, UL) the direction from the terminal to the base station In such cases a plurality of different transmission channels exist in either direction. With more modern mobile radio standards, such as the UMTS standard, there are two types of transport channel for the transmission of data: One type is the dedicated channels and the other is the common channels. A dedicated channel is reserved solely for the transmission of information for or from a specific terminal. Such a resource can for example be implemented by a specific frequency, or with systems operating in accordance with the CDMA (CDMA=Code Division Multiple Access) method, by using different spread codes on the same frequency. Information from the base station which is intended for all terminals can be transmitted on the common channels or the various terminals can share these channels, with each terminal only using the channel for a brief period.
It makes sense for the radio traffic within such a cell between the various terminals and the base station to be organized so that the base station is in a position, as regards the load imposed on it, to process all data and/or that the situation is avoided as far as possible in which different terminals are transmitting simultaneously on the same channels to the base station and collisions occur as a result. To this end which terminal which may send a message to the base station and the time at which it may send it should be defined in some way. The procedure mentioned at the start is executed for this purpose for example.
A typical example of such a selection procedure of a transmission channel is the random access procedure in the UMTS standard. This method is used to define which terminal may send a short message on which logical transmission channel to the base station, to inquire for example whether a call may be set up via the base station concerned or to send brief status information to the base station for example. The term “logical transmission channel” is in this case to be understood as the terminals using one and the same physical shared uplink channel, with the terminals employing different channelization codes. The base station can distinguish between the messages and signals sent by the various terminals on this physical common channel on the basis of the channelization codes, as if they had been sent on different channels. The common channel used is actually the “PRACH” (Physical Random Access Channel). In the current UMTS standard there are 16 different channelization codes currently available in the PRACH, i.e. 16 different logical transmission channels are implemented on the PRACH.
Before a mobile terminal uses the PRACH, on entering a specific cell for example, the terminal does not know which channelization codes are currently being used by other mobile terminals in this cell and which are free. As described at the start, the terminal therefore first sends to the base station a send authorization request signal—referred to in the UMTS standard as an access preamble. This access preamble is uniquely assigned to a specific channelization code or a specific logical transmission channel. The base station then sends a response signal, which either contains a confirmation that the device may send a message with this channelization code or with which the sending of the message with this channelization code is refused. Signaling is undertaken in this case with an individual decision value—in the UMTS standard in the form of an Acquisition Indicator. This Acquisition Indicator is sent on a specific downlink channel, known as the AICH (Acquisition Indicator Channel) from the base station to the terminals. The AICH—like the PRACH—is a Common Channel, which is to be received by all terminals. The Acquisition Indicator is multiplied on transmission by a specific signature character sequence, which in its turn is uniquely assigned to the channelization code concerned, for which the terminal has previously sent the send authorization request signal, so that the terminal concerned knows that it will be allowed or forbidden to send the message with this channelization code. With the current standard for executing the random access method the terminal receives via the common channel BCH (Broadcast Channel) which is permanently sent out by the base station of a specific cell, information about the channelization codes available as well as the necessary information as to when and if at all a send authorization request signal can be sent to the base station. From the potential channelization codes or logical transmission channels available for this mobile radio device, the mobile radio device then selects any given code and sends the access preamble for this specific transmission channel with a specific transmit power. If no response signal arrives after a specific time, it sends a send authorization request signal once again at increased power and for another transmission channel. If it then receives a response signal with a positive first decision value, it finally sends the message on the requested transmission channel—i.e. with specific channelization code on the PRACH—to the base station. For a negative decision value the procedure begins from the start, i.e. the terminal sends to the base station a further send authorization request for another transmission channel.
This method is acceptable provided it only relatively rarely occurs that the base station responds to an access preamble with a negative decision value, i.e. refuses the transmission on the transmission channel concerned. This is for example the case for methods in which a negative decision value is only sent if the base station is overloaded and cannot process any further data. Where the load on the base station continues to increase however or if a method is used in which specific transmission channels are also kept free for specific terminals for a specific time in order to prevent collisions, the number of the negative decision values increases significantly. In this case the method is ineffective since the terminal must request each individual possible transmission channel by sending out a suitable send authorization request signal.